Discussion

Calorimetry and so on.

Q = mcΔT

Burn a sample of the substance in oxygen, use the heat given off to warm a bit of water and watch the temperature rise.

When comparing fuels, it's often more instructive to speak of their energy density rather than their energy.

Energy Density of Selected Fuels
simple compounds	energy density (MJ/kg)		foods	energy density (MJ/kg)
carbon (to CO2)	32.8		carbohydrates	17.2
carbon (to CO)	22.7		fats	38.9
ethanol	29.7		proteins	17.2
hydrogen	142
methane	55.5		biomass fuels	energy density (MJ/kg)
methanol	22.7
nitromethane	11.6		alfalfa straw	18.4
octane	47.9		charcoal	29.0
propane	50.3		coconut husks	9.8
sulfur	9.16		coconut shells	17.9
			coffee husks	16.0
			cotton hulls	19.4
fossil fuels	energy density (MJ/kg)		cotton stalks	17.4
			dung, air dry	12.0
coal, anthracite	31.4		groundnut shells	19.7
coal, bituminous	> 23.9		maize cobs	18.9
coal, sub-bituminous	17.4–23.8		maize stalks	18.2
coal, lignite	< 17.43		peat	14.6
diesel	45.3		rice hulls	15.5
gasoline, automotive	45.8		rice straw	15.2
gasoline, aviation	43.1		sugar cane bagasse	19.0
jet a	43.3		sorghum bagasse	18.9
jet b	43.5		soybean stalks	19.4
kerosene	46.3		wheat straw	18.9
oil, crude (petroleum)	41.9		wood, green	10.9
oil, heating	42.5		wood, air dry	15.5
			wood, oven dry	20.0

Respiration (cellular respiration, that is, not pulmonary respiration or breathing as it is better known) is chemically identical to combustion, but it takes place at a much slower rate.

C₆H₁₂O₆ + O₂ → CO₂ + H₂O + energy

or in English

sugar combined with oxygen turns into carbon dioxide and water and releases energy

The energy content of foods is no longer measured directly much anymore. Instead, the food item in question is analyzed to determine the amount of protein, fat, and carbohydrate present in one serving. These numbers are then multiplied by the respective energy densities. In the US, where kilocalories are used instead of megajoules, the rule is 4-9-4: 4 kcal/g for protein, 9 kcal/g for fat, and 4 kcal/g for carbohydrate.

Adjustments are needed for indigestible fiber and specific energy factors / coefficient of digestibility.

Atwater system. Nutrition Labeling and Education Act of 1990.

Fuel-Based Energy Units
fuel unit	energy (MJ)	source
cubic meter natural gas (15 ℃, 1 atm)	37.1-38.7	Energy Information Administration
	37.3	Carbon Dioxide Information Analysis Center
	38.3	ConEdison (New York City)
	37.9	Niagara Mohawk (New York State)
	38.4	PSE&G (New Jersey)
toe ton of oil equivalent (1000 kg)	41,868	International Energy Agency
	42,622	Statistics Norway
	44,760	Carbon Dioxide Information Analysis Center
tce ton of coal equivalent (1000 kg)	29,308	International Energy Agency
	29,310	Center for Energy Efficiency
	29,290	Carbon Dioxide Information Analysis Center
ton of tnt	4,184	exact value by definition